Modulation in a transmitting apparatus for digital broadcast (for example, terrestrial digital broadcast in Japan) first maps a bit stream of a transmitting signal onto a complex plane through a mapping process. For example, in 64QAM (Quadrature Amplitude Modulation), a bit stream of a transmitting signal is segmented every 6 bits and mapped to 64 signal points on a complex plane.
Furthermore, in OFDM (Orthogonal Frequency-Division Multiplexing), the data (signal) mapped to 64 signal points on a complex plane is subjected to the IFFT (Inverse Fast Fourier Transform). Then, the digital signal having been subjected to the IFFT is converted to an analog signal, and then up-converted to a desired frequency and transmitted from an antenna.
On the other hand, demodulation in a receiving apparatus for terrestrial digital broadcast implements the opposite process to the above-described modulation. That is to say, an analog signal received by an antenna is converted to a digital signal, and furthermore, in OFDM, subjected to the FFT (Fast Fourier Transform) and propagation path equalization.
Furthermore, according to the above-described mapping arrangement, for example, in 64QAM, the signal after propagation path equalization is subjected to a demapping process to find the likelihood values (values to indicate which one of “0” and “1” is closer)) of the above 6 bits.
At this time, unlike the above-described case of modulation, noise, multiple paths (reflections of electromagnetic waves) and fading (frequency variation of electrical waves produced in reception during move) are produced on the propagation path, and therefore information that is deviated from the transmitting point is received.
Here, on I/Q coordinates (constellation) after propagation path equalization, detecting the transmitting point that is the closest a receiving point and assigning that transmitting point to each corresponding bit is a hard decision process, and quantizing the signal point assigned to each corresponding bit through the hard decision process by a plurality of bits and then performing decoding by finding how likely each bit subjected to the hard decision process is, is a soft decision process.
That is to say, in a hard decision process, every one bit is determined to be a binary signal of “0” or “1,” based on one threshold value, and, in a soft decision process, every one bit is determined as a multi-valued signal based on a plurality of threshold values.
As mentioned above, for example, in a digital broadcast demodulation circuit, a soft decision process to determine every one bit as a multi-valued signal based on a plurality of threshold values is performed.
Heretofore, to improve reception performance, the resolution (dividing values) of soft decision is increased by, for example, increasing the number of soft decision bits to use in the soft decision process.
However, increasing the number of soft decision bits results in an increased amount of information, which then causes an increase of circuit to perform process and power consumption and furthermore it becomes difficult to improve reception performance even if the number of soft decision bits is increased to a certain degree.
By the way, various digital broadcast receiving apparatuses (demodulation circuits) have been proposed heretofore.
Patent Document 1: Japanese Laid-open Patent Publication No. 2008-010987